This is the only kind of easel I have, and I like them very much.
Mine are made from black core double mat board. The only thing I don't like is that if I'm very close to printing the full frame, it can be hard to see if the edges are all excluded because the image is harder to see on black. It's not really a big problem, you just have to be careful.

If the bottom mat board is a non-actinic color such as orange, the image will still be plainly visible. There should be no light reflected back from the bottom board to degrade the image. Some older easels were white, and this could cause some fogging in the shadows, especially with thin paper.